1. Technical Field
The present invention is directed to the field of antenna systems for communication networks. More specifically, the invention provides an adaptive array antenna including at least one active element and one or more parasitic elements, in which the electrical loading of the parasitic elements is dynamically adapted based upon the content of the received radio frequency signal at the active element in order to optimize the antenna beam pattern. Antenna systems according to the present invention are particularly well-suited for use in high multi-path environments, such as within a cellular communication network in an urban area.
2. Description of the Related Art
Antenna pattern control (or beamforming) using antenna arrays has been implemented in may different forms for applications including wireless communications and radar. These include phased-array antennas, Butler matrices, non-adaptive analog and digital beamformers, switched beam antennas and fully-adaptive smart antennas.
Examples of fully-adaptive smart antennas, which are array antennas in which each of the antenna elements is an xe2x80x9cactivexe2x80x9d element, are shown in U.S. Pat. Nos. 4,639,914, 5,642,353, and in documentation for a publicly available system called xe2x80x9cIntelliCellxe2x80x9d from ArrayComm. These fully-adaptive antennas require active circuitry, i.e., a transmitter/receiver (TX/RX), on each of the antenna elements, and typically perform digital beamforming. The use of multiple transceivers (TX/RX) and digital beamforming dramatically increases the cost and complexity of these types of antenna systems, however, and thus limits their usefulness to those situations in which cost is not a key driver.
Parasitic array antennas include one or more active (or driven) elements and a plurality of parasitic elements. The active element is connected to a transceiver, but the parasitic elements are not. The electrical loading on the parasitic elements effect the radio frequency electromagnetic coupling between the parasitics and the active element(s), and hence the antenna beam pattern. Examples of parasitic array antennas include U.S. Pat. Nos. 4,700,197, 5,294,939, and 5,767,807. The antenna systems shown in these patents, however, do not provide a means for adaptively controlling the parasitic elements in order to match the electromagnetic to environment in which the antenna system is operating. Thus, these antennas are not suitable for use in environments that include a high degree of multi-path, such as in an urban environment.
In U.S. Statutory Invention Registration H26 and H375, an adaptive parasitic array antenna is disclosed. In this antenna system, the power level of the received signal at the active element is used to adaptively steer the beam pattern towards the highest received power level by adjusting the reactance (or loading) on the parasitic elements. Although such an antenna system may be useful for line-of-sight communications, such as in a missile tracking antenna system as disclosed in H26 and H375, it will not operate effectively in a high multi-path environment. In high multipath environments, the signal from a particular source (whether desired or interference) travels over many different paths due to scattering. The signal can arrive at the receiving antenna at many angles. Thus, forming distinct beampattern nulls to cancel interference and forming conventional high-gain lobes to admit the desired signal would be ineffective.
Secondly, these references adapt the parasitics based on the received power level only, and provide no mechanism for identifying the desired signal from the surrounding interference. Thus, the antenna system in H26 and H375 may steer the antenna beampattern to a high power level that is deplete of signal and contains simply interference. This is because H26 and H375 are only concerned with maximizing the power level of the signal, not its received quality. This is particularly problematic in high interference environments where the interference level can be equal to or greater than the signal level.
H26 and H375 are deficient in several other respects. They provide no teaching at all regarding the use of negative resistance devices as a loading element, either alone or in combination with reactive devices, in order to extend the beamforming capability of the parasitic elements. The references provide no detailed method for coordinating the control of the parasitic elements. They provide no teaching of separate acquisition and tracking modes, which, as described below, can be highly advantageous in an adaptive parasitic array antenna for use in dynamically changing environments. These references only relate to a receiving antenna system, and thus provide no teaching that relates to a transceiver antenna system that may both receive and transmit information. For these, as well as other reasons, H26 and H375 are highly limited in terms of an antenna system for use in a high multi-path, high interference environment. Indeed, neither of the structures shown in these references would work at all in such an interference rich, high multipath environment.
Thus, there remains a general need in this field for an adaptive parasitic array antenna system that is particularly well-suited for use in high multi-path environments.
An antenna system is provided including an adaptive parasitic array antenna comprising at least one active element and one or more parasitic elements coupled to controlled impedances (xe2x80x9cCIxe2x80x9d). The system further comprises a transceiver, a content-based optimization criterion computation module (xe2x80x9cCBOCCMxe2x80x9d), and a control variable optimizer (xe2x80x9cCVOxe2x80x9d). The CBOCCM receives a signal waveform from the active element through the transceiver, and computes an optimization criterion (xe2x80x9cOCxe2x80x9d) based on the content of the received signal. The optimization criterion is coupled to the CVO, which adaptively computes one or more control variables (xe2x80x9cCVxe2x80x9d), which are coupled to the controlled impedances in order to adjust the beampattern created by the adaptive parasitic array antenna. Also disclosed are two preferred adaptation implementations and algorithms, a pilot-tone based adaptation system, and a decision-directed based adaptation system.
By adapting the parasitic array antenna pattern based upon the content of the received signal (as distinguished from the power level or some other non-content based criterion), the present invention provides an antenna system that is capable of operating in high interference environments. An antenna system according to the present invention is particularly well-suited for use with cellular and other wireless communication systems that are deployed in urban areas where the environment is replete with multi-path. The system disclosed also provides a controlled impedance network for the parasitic elements that includes a negative resistance device, alone or in combination with a reactive device, in order to greatly extend the beamforming capabilities of the antenna.
One aspect of the invention provides an antenna system, comprising: an array antenna for generating a beam pattern, the array antenna comprising at least one active element and a plurality of parasitic elements, wherein the active element is coupled to a transceiver for transmitting and receiving data signals, and the parasitic elements are coupled to controlled impedance networks; and an adaptation controller coupled to the transceiver and the array antenna for extracting content information from the received data signals and altering the impedance of the controlled impedance networks in order to adapt the beam pattern of the array antenna.
Another aspect of the invention provides a method of operating an array antenna having at least one active element and a plurality of parasitic elements, the method comprising the steps of: (A) providing a plurality of controlled impedance networks coupled to each of the parasitic elements; (B) receiving a data signal at the array antenna; (C) extracting content information from the received data signal; and (D) altering the impedance of the controlled impedance networks based upon the content information so as to adapt the beam pattern of the array antenna.
Another aspect of the invention provides a system, comprising an array antenna having an active element and a plurality of parasitic elements, wherein each of the parasitic elements is coupled to a controlled impedance network; and a controller that receives a data signal from the array antenna and alters the impedance of the controlled impedance networks based upon the content of the data signal.
Still another aspect of the invention provides a pilot-tone based adaptive array antenna system, comprising an array antenna having at least one active element and a plurality of parasitic elements, wherein each of the plurality of parasitic elements is terminated with a controlled impedance network; a transceiver coupled to the active element for received a data signal from the array antenna and for transmitting a data signal to the array antenna; and an adaptation controller coupled between the transceiver and the plurality of parasitic elements, wherein the adaptation controller comprises an optimization criterion computation module for extracting a pilot tone signal from the received data signal and for generating an optimization criterion; and a control variable optimizer for generating a set of control variables based upon the optimization criterion, wherein the control variables are applied to the controlled impedance networks in order to adapt the beam pattern of the array antenna.
Still another aspect of the invention provides a decision-directed based adaptive array antenna system, comprising an array antenna having at least one active element and a plurality of parasitic elements, wherein each of the plurality of parasitic elements is terminated with a controlled impedance network; a transceiver coupled to the active element for received a data signal from the array antenna and for transmitting a data signal to the array antenna; and an adaptation controller coupled between the transceiver and the plurality of parasitic elements, wherein the adaptation controller comprises an optimization criterion computation module for generating an optimization criterion by comparing the received data signal with a reconstructed version of the received data signal; and a control variable optimizer for generating a set of control variables based upon the optimization criterion, wherein the control variables are applied to the controlled impedance networks in order to adapt the beam pattern of the array antenna.
Yet another aspect of the invention provides a method of operating an adaptive array antenna having at least one active element and a plurality of parasitic elements, wherein the plurality of parasitic elements are each coupled to a controlled impedance circuit, the method comprising the steps of: (A) providing a set of control variables; (B) setting the control variables to a mid-point value; (C) applying the control variables to the controlled impedance circuits; (D) operating the adaptive array antenna in an acquisition mode in which the values of the control variables are perturbed by a maximum amount; and (E) following the acquisition mode, operating the adaptive array antenna in a tracking mode in which the values of he control variables are perturbed by a minimum amount.
Another aspect of the invention provides an antenna, comprising: at least one active element; a plurality of parasitic elements; and a controlled impedance network coupled to each of the parasitic elements, wherein the controlled impedance network includes a tunnel diode.
The present invention overcomes the disadvantages of presently known parasitic array antenna systems and also provides many advantages, such as: (1) optimization for use in high multi-path environments; (2) provision for diversity combining and hence resilience to fading; (3) providing for a high degree of interference suppression; (4) providing for significant system channel capacity improvements through increased channel re-use; (5) providing adaptive directivity/antenna gain; (6) removing the need to physically point or re-point the antenna; (7) reducing portable terminal power consumption (over filly adaptive designs); (8) reducing cost of a wireless system deployment; (9) avoiding key cost drivers of fully adaptive antennas while achieving similar performance advantages; (10) the ability to be used in both base station and terminal equipment; (11) having a transmitting path through the beamforming antenna that is identical to the receiving path and hence no transmit-receive calibration is required; (12) reducing the requirement on channel equalization when used to suppress multi-path; and (13) in high interference environments, reducing receiver dynamic range requirements.
These are just a few of the many advantages of the present invention, which is described in more detail below in terms of the preferred embodiments. It should be noted that not all of these advantages are required in a system that practices the present invention and the listing is set forth only to illustrate the many possible advances that are provided. As will be appreciated, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects. Accordingly, the drawings and description of the preferred embodiments set forth below are to be regarded as illustrative in nature and not restrictive.